Plasma reactor having regions of active and passive electric field

ABSTRACT

A plasma reactor for automotive exhaust gas applications which efficiently promotes diffusion, mass transfer and chemical reaction processes of atoms, ions and radicals, in that the ground (outer) electrode has an axially discrete pattern which provides alternating regions of active and passive electric field along the axial direction of the plasma reactor. As the exhaust gas passes axially along the plasma reactor, each active region produces plasma atoms, ions and radicals, which then have time to react with the NO x  over the course of the adjacent passive region. In this manner, successive active regions produce copious atoms, radicals and ions, and the adjacent passive regions provide time for these radicals and ions to react with the NO x  and hydrocarbons before the next active region is encountered by the moving stream of exhaust gas, thereby enhancing the performance of the plasma reactor.

TECHNICAL FIELD

[0001] The present invention relates to plasma reactors used to reduceNO_(x) of internal combustion engine exhaust, and more particularly to aplasma reactor having nonuniform electric fields consisting of regionsof active and passive electric field along its length.

BACKGROUND OF THE INVENTION

[0002] The removal of nitrogen oxides (NO_(x)) from internal combustionexhaust is an increasing concern, especially for lean-burn engines suchas direct injection gasoline engines and Diesel engines. One method forpost combustion NO_(x) removal is the subjection of the exhaust gases toa non-thermal plasma process. In this regard, the exhaust gas is passedthrough a plasma processing tube whereat a high voltage electric fieldimparts formation of a plasma. The plasma has a large number ofenergetic electrons which collide with exhaust gas molecules to formatoms, ions and radicals These atoms, ions and radicals, in turn, reacteither with the NO to make NO₂, or with hydrocarbons to producealdehydes. The produced aldehhydes subsequently reduce NO₂ over suitablecatalysts to make harmless nitrogen. Thus, the major role of the plasmareactor is to produce NO₂ from NO and aldehydes from hydrocarbons in thecombustion exhaust stream. Among the aldehydes produced in the plasmareactor, acetaldehyde (CH₃CHO) is known to be the most effective for NO₂reduction over alkali-based catalysts.

[0003]FIGS. 1A through 1C depict three prior plasma reactors, whereinthe external high voltage source is either pulsating D.C. or A.C.

[0004]FIG. 1A depicts a first form of plasma reactor 10, referred tocommonly as a pulsed corona discharge plasma reactor, in which aconductive metallic tube 12 defines the reactor wall 14, and inside ofwhich exhaust gas G passes along. Axially along the concentric center ofthe tube 12 is a conductive high voltage electrode rod 16. The centralelectrode rod 16 is electrified by an external high voltage source withthe tube 12 serving as the ground electrode, wherein a corona is formedtherebetween without sparking which induces plasma formation of theexhaust gas.

[0005]FIG. 1B depicts a second form of plasma reactor 10′, referred tocommonly as a dielectric barrier discharge plasma reactor, in which aconductive metallic tube 22 and an insular dielectric layer 24, which isconcentrically disposed at the inside surface of the tube collectivelydefine the reactor wall 26. As in the first form of plasma reactor 10,exhaust gas G passes along the interior of the reactor wall 26, and aconductive high voltage electrode rod 28 is located axially along theconcentric center of the tube 22. The central electrode rod 28 iselectrified by an external high voltage source with the tube 22 servingas the ground electrode, wherein the dielectric layer 24 becomespolarized. The polarization of the dielectric layer 24 stores energywhich serves to aid the inducement of the plasma formation of theexhaust gas without sparking.

[0006]FIG. 1C depicts a third form of plasma reactor 10″, commonlyreferred to as a dielectric packed-bed discharge plasma reactor, inwhich, as in the second form of plasma reactor 10′, a conductivemetallic tube 32 and an insular dielectric layer 34, which isconcentrically disposed at the inside surface of the tube, collectivelydefine the reactor wall 36, wherein exhaust gas G passes along theinterior of the reactor wall 36, and a conductive high voltage electroderod 38 is located axially along the concentric center of the tube 32. Aplurality of small insular dielectric pellets 40 loosely fill theinterior of the reactor wall 36 such that the exhaust gas G is easilyable to travel through the spaces therebetween. The central electroderod 38 is electrified by an external high voltage source with the tube32 serving as the ground electrode, wherein the dielectric layer 34becomes polarized, and each of the pellets 40 becomes locally polarized,as well. The polarization of the dielectric layer 34 and of the localpolarization of the pellets 40 store energy which serves to aid theinducement of the plasma formation of the exhaust gas without sparking.

[0007] In the prior art, the plasma reactor wall may have either a flator cylindrical geometry, and the electrodes are typically made ofcontinuous electrical conductors, so that a uniformly active electricalfield is formed in the air gap therebetween to generate a plasma ofmaximum intensity for a given voltage. Prior art plasma reactorsemphasize production of a high intensity plasma based on an implicitassumption that the plasma intensity is the limiting factor of theunderlying process. The continuous electrodes utilized in the prior artplasma reactors may be suitable for operating conditions where thesupply of high energy electrons is the rate limiting step of the plasmareaction. However, when the rate limiting step is other than theelectron supply, an increase in input energy above a certain valuethrough the continuous electrodes will hardly improve the overallperformance of the plasma process.

[0008] The inventors of the present invention, while investigatingplasma assisted lean NO_(x) catalysis, have discovered that the limitingfactor of the plasma reaction process is not the intensity of the plasmabut the diffusion, mass transfer and chemical reaction of intermediates(such as atoms, ions and radicals) produced in the plasma under theoperating conditions of a typical automotive engine exhaust gas stream.Thus, it is important to promote the diffusion, mass transfer andchemical reaction processes of atoms, ions and radicals in the plasmareactor in order to improve the overall performance of the NO_(x)reduction process in the engine exhaust. In this regard, it is notedthat energy is invested in the dielectric layer of prior art plasmareactors without an efficient pay-out with respect to the plasma energyin terms of encouraging maximal reaction of the atoms, ions and radicalswith respect to the NO_(x) and hydrocarbons.

[0009] Accordingly, what remains needed in the art of plasma reactors isto somehow provide an operative configuration which efficiently promotesdiffusion, mass transfer and chemical reaction processes of atoms, ionsand radicals in a plasma generator for automotive exhaust gasapplications.

SUMMARY OF THE INVENTION

[0010] During a study by the inventors hereof of plasma assisted leanNO_(x) catalysis using simulated engine exhaust gases, identified wasthe mass transfer/chmeical reaction rate of ionized reactant species(not the electron supply) as the rate limiting step of the plasmaprocess under typical operating conditions. It is the present inventors'discovery that an increase of mass transfer/chemical reaction rate ofthe ionized reactants can be achieved by arranging the ground electrodein discrete locations along the axial length of the plasma reactor,which thereby provides passive regions therebetween serving as effectivemass transfer/chemical reaction areas.

[0011] The present invention is a plasma reactor for automotive exhaustgas applications which efficiently promotes diffusion, mass transfer andchemical reaction processes of ions and radicals, in that the ground(outer) electrode has an axially discrete pattern, as for example aspiral pattern, which provides alternating regions of active and passiveelectric field. As the exhaust gas passes axially along the plasmareactor, each active region produces atoms, ions and radicals due to theplasma reaction, which then have time to react with the NO_(x) andhydrocarbons over the course of the adjacent passive region. In thismanner, successive active regions produce copious atoms, radicals andions, and the adjacent passive regions provide time for these atoms,radicals and ions to react with the NO_(x) before the next active regionis encountered by the moving stream of exhaust gas. Because each activeregion is axially compact, an intense generation of energetic electronsoccurs thereat; because each passive region is axially extended,sufficient time is provided for the resulting ions and radicals to reactwith the NO_(x) before the next active region is encountered.Accordingly, the energy consumption in relation to the production of NO₂and aldehydes is extremely favorable.

[0012] This favorable result can be explained as follows. Since the ionand radical velocity depends on the strength of the electric field,these velocities in the radial direction (transverse to the axialdirection) are faster in the active regions than in the passive regions.This velocity gradient in the radial direction, with the help of thespatially alternating electric field, promotes axial mixing of ionizedreaction intermediates, resulting in an enhanced reactivity with theNO_(x) and hydrocarbons. This mixing, in turn, means that the plasmareactor according to the present invention requires much less energy tooperate than the prior art plasma reactors to achieve the same level ofperformance.

[0013] In a preferred embodiment, the plasma reactor has a cylindricalgeometry, having a plasma reactor wall composed of a dielectric tube. Ahigh voltage (inner) electrode rod is disposed at the concentric centerof the dielectric tube. A ground (outer) electrode is connected toground and is in the form of a wire is coarsely wound around thedielectric tube to provide a sequential pattern composed of a series ofdiscretely spaced apart locations. Exhaust gas passes axially along thedielectric tube. An alternating high voltage is applied to the centrallydisposed electrode rod, and the ground is connected to the groundelectrode.

[0014] The efficiency of the plasma reactor according to the presentinvention to reduce NO_(x) in automotive exhaust gas is greatly improvedover prior art plasma reactors due to:

[0015] elimination of unnecessary power consumption which can furthercontribute to the overall performance of the plasma reactor, leading tothe minimum power consumption;

[0016] overall mass transfer rate of ionized reactants enhancement dueto the passive regions serving as areas for promoting diffusion and masstransfer of these reactants;

[0017] spatially oscillatory electric field inducement by the alternatesequence of active and passive regions of the electric field promotingmixing among ionized reactant species, and resulting in an enhancedreaction rate in the plasma reactor; and

[0018] passive regions of the electric field providing an increasedeffective capacitance of the dielectric material constituting the plasmawall, tending to increase the input power for a given voltage, andtranslating to a decrease of the high voltage necessary to obtain adesirable input power.

[0019] Accordingly, it is an object of the present invention to providea plasma reactor having sequentially alternating active and passiveelectric field regions axially therealong.

[0020] This and additional objects, features and advantages of thepresent invention will become clearer from the following specificationof a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIGS. 1A through 1C are partly sectional side views of prior artplasma reactors;

[0022]FIG. 2 is a partly sectional side view of a plasma reactoraccording to the present invention;

[0023]FIG. 3 is a sectional end view, seen along line 3-3 of FIG. 2;

[0024]FIG. 4 is a schematic diagram of an electrical circuit for theplasma reactor according to the present invention;

[0025]FIG. 5 is a graphical plot of axial distance versus electric fieldintensity of the plasma reactor according to the present invention;

[0026]FIGS. 6A and 6B are Lissajous diagrams for a prior art plasmareactor (FIG. 1B) and a plasma reactor according to the presentinvention (FIG. 2), respectively, which can be used to estimate theeffective capacitance of the dielectric barrier respectively thereof;

[0027]FIG. 7 is a graphical plot of input energy versus NO_(x)conversion (reduction) for a prior art plasma reactor (FIG. 1B) and aplasma reactor according to the present invention (FIG. 2);

[0028]FIG. 8 is a schematic representation of a first form of acommercial plasma reactor according to the present invention;

[0029]FIG. 9 is a schematic representation of a second form of acommercial plasma reactor according to the present invention; and

[0030]FIG. 10 is an exhaust gas treatment system composed of the plasmareactor according to the present invention and a catalytic converter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031] Referring now to the Drawing, FIGS. 2, 3, 4, 5, 6B and 7 depictvarious aspects of an example of an automotive exhaust gas plasmareactor 100 according to the present invention, wherein seriallyalternating regions of active and passive electric field are providedalong its axial length; and FIGS. 8 and 9 depict first and secondcommercial versions of the plasma reactor according to the presentinvention.

[0032] The plasma reactor 100 according to the present invention has anelongated cylindrical configuration defined by a plasma reactor wall 102composed of an insular dielectric material 102′ which serves as adielectric barrier and defines a reactor space thereinside. Thecomposition of the plasma reactor wall 102 may be any suitabledielectric material, as for example quartz, glass, alumina, etc. By waymerely of exemplification and not limitation, the dielectric materialmay be a quartz tube having a three-eighths inch outside diameter and awall thickness of 1 mm. Each end of the plasma reactor wall 102 issealingly closed by a respective end wall 104 e, 104 e′. An inlet 106 islocated at one end of the plasma reactor wall 102, and an outlet 108 islocated at the other end of the plasma reactor wall.

[0033] A high voltage electrically conductive central (inner) electroderod 110 is located at the concentric center of the plasma reactor wall102. The central electrode rod 110 extends at least as long as theplasma wall, preferably being anchored by passage (in sealing relation)through each of the end walls 104 e, 104 e′. The central electrode rod110 may be composed of any suitable electrically conductive material, asfor example stainless steel, aluminum, copper, etc. By way merely ofexemplification and not limitation, the central electrode rod 110 may becomposed of stainless steel, having a diameter of one-eighth inch. Thecentral electrode rod 110 is connected to the high voltage output of atime variant high voltage source 112 (see FIG. 4). A dielectric materialmay cover the central electrode rod, for example for the purpose ofenvironmental protection.

[0034] An electrically conductive ground (outer) electrode 114 is in theform of a wire 114′ (by “wire” is meant to include all equivalentsthereof, as for example narrow, thin electrically conducting film) whichis wound tightly (contactingly) around the plasma reactor wall 102,having a selected pitch which provides an axially discrete spacing d. Inthis regard, the ground electrode is arranged in a sequential patterncomprising a series of discretely spaced apart locations, the sequentialpattern being arranged with respect to the axis A along which theexhaust gas G flows. The pitch may be such as to provide coarse spacing(preferred), close spacing, constant spacing (preferred) or variablespacing. The ground electrode 114 is connected to ground of the highvoltage source 112. By way merely of exemplification and not limitation,the ground electrode 114 may be composed of small gauge copper/nickelwire coiled tightly around the plasma reactor wall 102.

[0035] The outer surface area of the plasma reactor wall 102 which is inlocal contact with the (wire 114′ of the) ground electrode 114 definesan active region 116 of the electric field formed by the voltaicinteraction between the electrode rod 110 and the ground electrode,wherein the electric field is moderated by polarization of thedielectric material of the plasma reactor wall 102. The space betweenthe active regions 116 axially along the plasma reactor wall 102constitute passive regions 118 of the electric field. The active regions116 of the electric field have higher field intensity than the passiveregions 118, wherein the resulting axially varying electric field isrepresented by the plot 120 of FIG. 5.

[0036] In FIG. 5, the axial locations 120 a of the ground electrode andits associated active region of electric field intensity are shown bythe rectangular solid line bars. The nonuniform electric field intensity120 b is shown by the dashed lines. The smooth transition of thenonuniform electric field intensity between the active and passiveregions 116, 118 is due to the free drift of high energy electrons fromthe active regions to the passive regions, which results from thephysical nature of the free boundary between the active and passiveregions.

[0037] In operation of the plasma reactor 100 with respect to automotiveexhaust gas G, as the gas passes into the plasma reactor via the inlet106, a plasma is formed by the voltage across the central electrode rod110 and the ground electrode 114, moderated by polarization of thedielectric material 102′, wherein diffusion, mass transfer and chemicalreaction processes of atoms, ions and radicals are promoted because theground electrode has an axially discrete pattern due to its pitchedcoiling. In this regard, the pitched coiling provides sequentiallyalternating short axial distance active regions 116 and long axialdistance passive regions 118 of the electric field (see FIGS. 2 and 5).By way of illustration and not limitation, the ratio of the axialdistance of an active region in relation to the axial distance of apassive region may be about an order of magnitude. As the exhaust gas Gpasses axially along the plasma reactor from the inlet 106 to the outlet108, each encountered active region 116 produces plasma atoms, ions andradicals of the exhaust gas, which then have time to react with theNO_(x) and hydrocarbons of the exhaust gas over the course of thedownstream adjacent passive region 118. In this manner, successiveactive regions produce copious atoms, radicals and ions, and theadjacent passive regions provide time for these atoms, radicals and ionsto react with the NO_(x) and hydrocarbons before the next active regionis encountered by the moving stream of exhaust gas.

[0038] It has been determined that optimization of the active andpassive regions of the electric field in a plasma reactor 100 accordingto the present invention can be represented by the relation:

d h·cos(π/3),  (1)

[0039] wherein d (as for example shown at FIG. 2) is the distancebetween the mutually adjoining active and the passive regions (i.e., theaxial length of the passive regions), and wherein h (as for exampleshown at FIG. 3) is the shortest distance between the central electroderod and the outer ground electrode. The exact value of d depends on thespecific kinetics of the chemical reaction occurring in the plasmareactor.

[0040] An experiment demonstrating a successful implementation of theplasma reactor 100 was conducted using a simulated engine exhaust gasmixture. Observed was a bright annular region between the centralelectrode rod and the (quartz tube) dielectric plasma reactor wall,which brightness represented the exhaust gas plasma. On the outersurface of the plasma reactor wall, observed were bright regionsadjacent to the spiraling wire of the outer ground electrode, whichrepresented the active regions of the electric field. Also observed onthe outer surface of the plasma reactor wall were dark regions betweenthe bright regions, which represented the passive regions of theelectric field. Despite this clear distinction between the active andpassive regions of the electric field as shown on the outer surface ofthe (dielectric barrier) plasma reactor wall, the plasma intensity inthe annular region was observed to be almost uniform, indicating a goodgas phase mixing in the plasma. This result can be explained as follows.Since the ion velocity depends on the strength of the electric field, itis faster in the active regions of the electric field than in thepassive regions of the electric field. This, with the help of thespatially alternating electric field, promotes axial mixing of ionizedreaction intermediates, resulting in an enhanced reactivity of theNO_(x).

[0041]FIGS. 6A and 6B are Lissajous diagrams for a prior art plasmareactor, as shown at FIG. 1B, and a plasma reactor according to thepresent invention, as shown at FIG. 2, respectively, which areessentially phase diagrams of voltage vs. charge. In both cases, an ACvoltage of +/−10.6 KV in a sine wave was applied to the centralelectrode at a frequency of 200 Hz. It should be noted that the scale ofthe x-axis is vastly different between FIGS. 6A and 6B. The nearlyperfect parallelograms indicate constant dynamic capacitance, and theslope of line AB, A′B′ (or line CD, C′D′) correspond to reciprocalcapacitance of the dielectric barrier (1/C_(b)), the same dielectricbarrier being used for both plasma reactors. The effective capacitanceof the dielectric barrier (C_(b)) estimated from FIGS. 6A and 6B are:C_(b)=10 pF for the prior art plasma reactor, and C_(b)=53 pF for theplasma reactor according to the present invention. It is remarkable thatthe effective capacitance of the same dielectric barrier can beincreased by 530% by the plasma reactor of the present invention,compared to the prior art plasma reactor. The total electrical energyabsorbed by the gas plasma (P) was estimated from the area of theparallelogram. The results are: P=0.18 watt for the prior art plasmareactor, and P=1.02 watt for the plasma reactor according to the presentinvention. Clearly, the plasma reactor of the present invention absorbsthe electrical energy much more efficiently than the prior art plasmareactor.

[0042] The increase in C_(b) is of practical significance, because theelectrical power absorbed by the gas plasma (P) is proportional to C_(b)according to the following equation:

P=4f·E _(f) ·C _(b) ·[V _(m) −E _(f)·(C _(g) +C _(b))/C _(b)],  (2)

[0043] wherein, P is the input power (watts), f is the frequency (s⁻¹),E_(f) is the constant voltage drop across the gap when conducting(volts), V_(m) is the peak voltage (volts), C_(b) is the dielectricbarrier capacitance (Farads), and C_(h) is the gap capacitance (Farads).

[0044] Table 1 compares several performance aspects of plasma reactorsof the prior art (FIG. 1C) and of the present invention (FIG. 2) for ahigh voltage frequency of 200 Hz. The feed gas to the respective plasmareactors contained 225 ppm NO, 600 ppm C₃H₆, 12% O₂, 2.5% H₂O, and thebalance N₂. Clearly, the present invention is much more efficient inconverting hydrocarbons (CH₃CHO) to acetaldehyde which is the mostactive NO_(x) reductant. Also, the electrical energy is shown to beabsorbed more efficiently at lower voltage by the present invention thanby the prior art. TABLE I Performance Aspect Prior Art Present InventionC₃H₆ CH₃CHO 11% 20% NO NO₂ 100% 100% C₃H₆ conversion 25% 60% Peakvoltage used  9 kV 5.5 kV Energy (E) absorbed 15 J/L  20 J/L

[0045]FIG. 7 demonstrates the improved performance of the plasma reactor100 with respect to NO_(x) reduction (conversion), plot 122, as comparedwith a prior art plasma reactor having a continuous outer groundelectrode (FIG. 1B), plot 124. A simulated exhaust gas mixturecontaining 75 ppm NO, 200 ppm C₃H_(6, 12)% O₂, 2.5% H₂O and balance N₂was fed first to the plasma reactor and then to the catalytic reactorcontaining Na/zeolite Y catalysts. Both plasma reactors were at 25 C andsubjected to an AC voltage in a sine wave with amplitude of 10.6 KV andfrequency of 200 Hz. The temperature of the catalytic reactor was keptat 180 C. Clearly, the plasma reactor according to the present inventionperforms much better than that of the prior art plasma reactor for thesame input energy to the plasma, when used in series with a Na/zeolite Ycatalyst. Accordingly, the plasma reactor 100 according to the presentinvention advantageously requires much less energy than a prior artplasma reactor to achieve the same performance of the plasma assistedNO_(x) reduction (conversion). Additionally, the plasma reactoraccording to the present invention is more stable and durable over awide range of over-voltages than prior art plasma reactors.

[0046] For commercial implementation of the plasma reactor 100 amulti-unit, juxtaposed construction, similar to a typical heat exchangerdesign, would be advantageous.

[0047] In a first commercial form of commercial plasma reactor 140according to the present invention, shown at FIG. 8, a plurality ofjuxtaposed cylindrical plasma reactors 100′, each having a cylindricaldielectric plasma reactor wall 102 a forming a reactor spacethereinside, a centrally disposed high voltage central electrode rod 110a and an outer coiled wire ground electrode 114 a, as structurally andoperatively recounted hereinabove with respect to FIGS. 2 through 4.With regard to the circuit of FIG. 4, the central electrode rods 110 aare all wired together in parallel, and the ground electrodes 114 a arealso all wired together in parallel. A cylindrical chamber 142 containsthe plurality of plasma reactors 100′, each spaced apart and supportedat each end by entry and exit end chambers 144 a, 144 b. An entry pipe146 a provides exhaust gas G entry into the entry end chamber 144 a,which, in turn, effects communication between the entry pipe and theplurality of plasma reactors 100′. An exit pipe 146 a (which connects tothe vehicle's catalytic converter, see FIG. 10) provides exiting of theexhaust gas G from the plurality of plasma reactors 100′, wherein theexit end chamber 144 b effects communication between the exit pipe andthe plurality of plasma reactors. The exhaust gas travels only insidethe plasma reactors between the entry and exit end caps.

[0048] In a second commercial form of commercial plasma reactor 150according to the present invention, shown at FIG. 9, a stack ofjuxtaposed rectangular plasma reactors 100″ are located within arectangular chamber 152. Each plasma reactor 100″ has a dielectricplasma reactor wall 102 b in the form of two superposed, mutuallyparallel and spaced apart flat dielectric sheets defining a reactorspace therebetween, a centrally disposed (inner) high voltage electrodesheet 110 b (also flat), and an outer ground electrode 114 b composed ofwire in the form of a series of mutually interconnected (using forexample an end shunt S, or a zig-zag arrangement), and mutually spacedapart, wire segments laying across the plasma reactor wall transverselyor obliquely in relation to the axial direction. Mutually adjacentplasma reactors 100″ may share the ground electrode 114 b. Theelectrodes are wired as indicated in FIG. 4, wherein the electrodesheets 110 b are all wired together in parallel, and the groundelectrodes 114 b are also all wired together in parallel. The flatsheets (dielectric plasma reactor wall 102 b and central electrode 110b) are supported at each end by entry and exit end chambers 154 a, 154b. An entry pipe 156 a provides exhaust gas G entry into the entry endchamber 154 a, which, in turn, effects communication between the entrypipe and the plurality of plasma reactors 100″. An exit pipe 156 a(which connects to the vehicle's catalytic converter, see FIG. 10)provides exiting of the exhaust gas G from the plurality of plasmareactors 100″, wherein the exit end chamber 154 b effects communicationbetween the exit pipe and the plurality of plasma reactors. The exhaustgas travels only inside the plasma reactors between the entry and exitend caps. The electrode sheets 110 b may be covered by a dielectricmaterial for example for the purpose of environmental protection.

[0049]FIG. 10 depicts schematically an exhaust gas treatment system 160composed of a plasma reactor 100 upstream of a catalytic converter 162.After treatment of the exhaust gas G by the plasma reactor 100, thecatalytic converter 162 further treats the exhaust gas. A catalyst, asfor example NaY, is in the catalytic converter.

[0050] To those skilled in the art to which this invention appertains,the above described preferred embodiment may be subject to change ormodification. For example, the plasma reactor wall may have any suitablegeometry. Such change or modification can be carried out withoutdeparting from the scope of the invention, which is intended to belimited only by the scope of the appended claims.

1. A plasma reactor for making NO₂ from NO and aldehydes fromhydrocarbons in automotive exhaust gas, comprising: a dielectric plasmareactor wall, said dielectric plasma reactor wall defining a reactorspace; an inner electrode disposed within said reactor space in parallelrelation to said dielectric plasma reactor wall; and an outer electrodeadjoining said dielectric plasma reactor wall, said outer electrodebeing arranged in a sequential pattern comprising a series of discretelyspaced apart locations, said sequential pattern being arranged withrespect to an axis.
 2. The plasma reactor of claim 1, further comprisinga source of time varying high voltage connected to said inner and outerelectrodes.
 3. The plasma reactor of claim 1, wherein said outerelectrode is comprised of electrically conductive wire and equivalentsthereof.
 4. The plasma reactor of claim 3, wherein said inner electrodeis disposed substantially centrally and coextensively with respect tosaid dielectric plasma reactor wall.
 5. The plasma reactor of claim 4,further comprising a source of time varying high voltage connected tosaid inner and outer electrodes; wherein said source provides, withrespect to said inner and outer electrodes and said dielectric plasmareactor wall, sequentially alternating regions of active and passiveelectric field in a direction along said dielectric plasma reactor wallparallel to the axis.
 6. The plasma reactor of claim 5, wherein eachsaid region of active electric field has an active distance parallel tothe axis which is shorter than a passive distance parallel to the axisof any said passive region of electric field, substantially defined by arelation: d h·cos(π/3), wherein d is a length of the regions of passiveelectric field parallel to the axis, and wherein h is a shortestdistance between said inner electrode rod and said outer electrode. 7.The plasma reactor of claim 6, wherein said sequentially alternatingregions of active and passive electric field provide an effectivecapacitance of said plasma reactor which exceeds an intrinsiccapacitance of said plasma reactor.
 8. The plasma reactor of claim 7,wherein said dielectric plasma reactor wall has a cylindrical geometry,and wherein said wire is wound spirally with respect to the axis aroundsaid dielectric plasma reactor wall.
 9. The plasma reactor of claim 7,wherein said dielectric plasma reactor wall comprises a pair ofsuperposed, mutually parallel and spaced apart flat dielectric sheets,wherein said inner electrode comprises a flat conductive sheet mediallypositioned between the flat dielectric sheets, and wherein said wirecomprises a plurality of mutually spaced apart and shunted wire segmentsadjoining each of the flat dielectric sheets, the wire segments beingarranged at an orientation selected from the group consisting of:transversely with respect to the axis and obliquely with respect to theaxis.
 10. A multi-unit plasma reactor for reducing NO_(x) in automotiveexhaust gas, comprising: a plurality of mutually juxtaposed plasmareactors, each plasma reactor comprising: a dielectric plasma reactorwall, said dielectric plasma reactor wall defining a reactor space; aninner electrode disposed within said reactor space in parallel relationto said plasma reactor wall; and an outer electrode adjoining saidplasma reactor wall, said outer electrode being arranged in a sequentialpattern comprising a series of discretely spaced apart locations, saidsequential pattern being arranged with respect to an axis.
 11. Theplasma reactor of claim 10, wherein each said plasma reactor furthercomprises: said dielectric plasma reactor wall having a cylindricalgeometry; and said wire being wound spirally with respect to the axisaround said dielectric plasma reactor wall.
 12. The plasma reactor ofclaim 10, wherein each said plasma reactor further comprises: saiddielectric plasma reactor wall comprising a pair of superposed, mutuallyparallel and spaced apart flat dielectric sheets; said inner electrodecomprising a flat conductive sheet medially positioned between the flatdielectric sheets; and said wire comprising a plurality of mutuallyspaced apart and shunted wire segments adjoining each of the flatdielectric sheets, the wire segments being arranged at an orientationselected from the group consisting of: transversely with respect to theaxis and obliquely with respect to the axis.
 13. A treatment system forautomotive exhaust gas, comprising: at least one plasma reactorcomprising: a dielectric plasma reactor wall forming a reactor space,said dielectric plasma reactor wall having an inlet and an outlet forexhaust gas entry and exit, respectively, with regard to said reactorspace; an inner electrode disposed within said reactor space in parallelrelation to said dielectric plasma reactor wall; and an outer electrodeadjoining said plasma reactor wall, said outer electrode being arrangedin a sequential pattern comprising a series of discretely spaced apartlocations, said sequential pattern being arranged with respect to anaxis; and a catalytic converter connected to said outlet of said atleast one plasma reactor.
 14. The treatment system of claim 13, whereinsaid at least one plasma reactor comprises a plurality of mutuallyjuxtaposed said plasma reactors.
 15. The treatment system of claim 14,wherein each said plasma reactor further comprises: said dielectricplasma reactor wall having a cylindrical geometry; and said wire beingwound spirally with respect to the axis around said dielectric plasmareactor wall.
 16. The treatment system of claim 14, wherein each saidplasma reactor further comprises: said dielectric plasma reactor wallcomprising a pair of superposed, mutually parallel and spaced apart flatdielectric sheets; said inner electrode comprising a flat conductivesheet medially positioned between the flat dielectric sheets; and saidwire comprising a plurality of mutually spaced apart and shunted wiresegments adjoining each of the flat dielectric sheets, the wire segmentsbeing arranged at an orientation selected from the group consisting of:transversely with respect to the axis and obliquely with respect to theaxis.
 17. A method for reducing NO_(x) from automotive exhaust gashaving hydrocarbons, comprising the steps of: providing a stream ofexhaust gas moving generally parallel with respect to an axis;subjecting the gas to a region of high voltage extending for a shortdistance parallel to the axis; subjecting the gas immediately thereafterto voltage lower than said high voltage for a long distance parallel tothe axis; and sequentially repeating a selected number of times saidfirst and second steps of subjecting; wherein atoms, radicals and ionsare produced in the gas by the first step of subjecting, and whereinNO_(x) in the gas is reduced by interaction of the NO_(x) with theatoms, ions and radicals during said second step of subjecting.
 18. Themethod of claim 17, wherein said second step of subjecting comprises NOof the NO_(x) in the exhaust gas oxidizing into NO₂; and thehydrocarbons oxidizing into aldehydes by interaction with the atoms,ions and radicals.